In aircraft applications it is highly desirable to minimize the weight of aircraft components as every pound saved in aircraft weight translates to fuel savings and/or greater payload capacity. With respect to propeller, turboprop or turbofan aircraft engine components, it is well appreciated that the propulsor blades are the most likely candidate for weight reduction since the weights of other related components, e.g. blade retention means, pitch change mechanisms, hub disks, shafts and bearings, are typically directly dependent upon the magnitude of the blade centrifugal loading borne by these components. The propulsor blades per se, however, can be made lighter in weight by manufacturing the blades in a spar and shell construction wherein the blade is formed of outer shell made of lightweight composite material, such as fiber reinforced resin, and an internal load bearing spar which is typically a metallic or composite member bonded to the interior surface of the shell and extends from within the shell cavity to terminate beyond the shell in a root end which is adapted to be mounted to a suitable blade retention means. Examples of such spar and shell construction blades are presented in commonly assigned U.S. Pat. Nos. 4,470,862; 4,648,921; and 5,042,968.
It has become conventional practice in the aircraft industry to manufacture such spar and shell blades with the shell formed about the load bearing spar as a molded fiber reinforced resin body formed by layers of fabric impregnated with resin, commonly via resin transfer molding methods, and cured in a mold contoured to the airfoil shape desired for the blade. Such fiber reinforced resin shells exhibit high strength and low weight characteristics and in aircraft applications typically offer at least as high strength as corresponding articles made of metal at a substantially lower weight.
For example, commonly assigned U.S. Pat. No. 4,648,921 discloses a method of making a fiber reinforced airfoil shaped propeller blade assembly wherein 4 to 7 layers of woven fiberglass cloth are layed up over a spar/foam underbody comprising a full length metallic spar having foam leading and trailing edges. The spar/foam underbody is formed by injecting a lightweight foam material into a mold disposed about an adhesive coated full length metallic spar and suitably curing the foam. A particular method of making such a spar/foam underbody for a propulsor blade is disclosed in commonly assigned
U.S. Pat. No. 5,042,968. After curing, the spar/foam underbody is wrapped in multiple layers of fibrous cloth, such as fiberglass cloth, each of the fiberglass layers being trimmed to its desired contour and then hand stitched in place over the underbody. Alternately, as disclosed in commonly assigned U.S. Pat. No. 4,470,862, the hand stitching may be eliminated by adhesively bonding each fiberglass layer to the layer therebeneath. To do so, the fiberglass material is provided on its underside with a minute, but effective, amount of thermoplastic adhesive. The material is then trimmed to shape and placed in position over the subassembly. Thereafter the adhesive is activated by heat and pressure by means of an electric resistance heated hand iron applied to the surface of the fiberglass material. In either case, this cloth wrapped assembly is then placed in a second mold and a synthetic polymeric material, such as epoxy resin, is injected into the fiber matrix and then cured. Alternatively, the resin may be applied to the fibrous cloth of the wrapped subassembly to preimpregnate the fibrous cloth before it is placed into the curing mold.
A light-weight rotary machine blade comprising a composite spar, formed of a partial length metal root and a spanwisely extending foam body wrapped in an aramid fiber wrap reinforced with high strength graphite plies, and a surrounding fiber-reinforced composite shell is disclosed in commonly assigned, U.S. Pat. No. 5,269,658, of John A. Violette and Charles E. K. Carlson. Also disclosed therein is a method for manufacturing such a composite blade comprising the steps of: installing an elongated core of lightweight cellular foam material into a receiving cavity defined by the flared distal end of a foreshortened metal spar so as to extend axially outwardly therefrom, thence laying up a laminate fiber wrap of alternating plies of spanwisely oriented graphite fibers and angularly woven plies of high strength aramid fibers thereabout to form a spar subassembly, thence attaching leading and trailing edge fillers of lightweight foam material to the composite spar subassembly to form the desired contoured shape of the blade, thereafter laying up a laminate wrap of layered plies of high strength aramid fibers about the shaped spar/foam subassembly except for the root end of the spar, and thence placing the wrapped spar/foam subassembly into a conforming mold and impregnating the wrapped spar/foam subassembly with an epoxy resin via resin transfer molding techniques to yield a resin reinforced assembly which upon curing constitutes the lightweight composite blade.
In such lightweight propulsor blades of spar and shell construction, the spar comprises the primary loading bearing member for effectively transmitting the centrifugal pull, bending moments, torsion loads and vibratory loads imposed upon the blades during operation to the blade retention means for distribution to the load carrying blade retention structure and the hub into which the blades are mounted. However, the shell of the blade, in addition to forming the desired airfoil shape of the propulsor blade, also participates in transmitting a portion of these loads imposed upon the propulsor blade to the load carrying blade retention structure. Unlike the spar, the shell is exposed to the environment and therefore susceptible to damage in the event that the propulsor blade is struck by a foreign object, for example a bird, a rock or the like. Accordingly, it is customary to protect the leading edge of such a fiber reinforced resin shell with a protective metallic sheath, typically made of nickel or titanium or alloys thereof, to absorb the impact energy imparted to the blade by a foreign object strike and prevent, or least lessen, damage to the foam leading edge filler disposed beneath the shell and about the leading edge of the spar.